1. Field of the Invention
The present invention relates to a rotary screen printing cylinder comprising a cylindrical screen having at least one zone provided with color-permeable openings at least in partial areas, a squeegee disposed within the screen, and two end pieces each fastened to an end of the screen as an extension of the cylindrical surface. The invention relates further to a system of such rotary screen printing cylinders and to a method for producing them.
2. Description of the Prior Art
In screen process printing, the color is pressed with the help of a squeegee through a stretched screen surface, generally a plastic or metal fabric of predetermined mesh width, the fabric being impervious to color in the nonprinting areas. The pores are usually blocked photographically by the screen material being coated with a photographic emulsion and exposed to the desired composition pattern. The exposed areas are washed away during the subsequent development of the emulsion so that the screen becomes color-permeable on the exposed areas.
Originally a screen printing unit comprises a metal or wooden frame over which the screen material is stretched without deformation. Due to the economic necessity of producing faster and more efficient printing machines one has developed rotary screen printing units which allow continuous printing of endless sheets. The screen is no longer flat but formed as a cylinder envelope inside which the color squeegee is fastened.
The cylindrical screen is produced for example by accordingly shaping a plastic or metal fabric which is subsequently welded along the cylindrical surface. However this weld causes problems during printing, so that rotary screen printing cylinders are very frequently electroplated as this production process obtains a uniform screen surface.
EP 0 164 149 describes such a production method. In this method an electroconductive screen master is placed in an electrolytic bath and connected with the cathode of a current transmitter. A metal layer is then deposited on the master, which is subsequently removed from the master and patterned. Prior to patterning the screen material has a uniform perforation which must be made color-impermeable in the nonprinting areas for printing. This is usually done with the help of the abovementioned photographic methods.
EP 0 338 612 A1 describes, for example, such a method for rotary screen printing cylinders. The screen likewise consists in this case of a perforated metal cylinder to which an end piece is glued as an extension of the cylindrical surface in each case prior to patterning. The rigidly mounted end pieces allow easy and quick replacement of the printing cylinder in the printing unit, which is of great benefit particularly with small print orders. Only after the connecting elements are glued to the screen is the latter coated with the photographic emulsion and exposed in the usual way. As soon as a new printing pattern is required the unexposed photographic emulsion, which blocks the nonprinting areas of the uniformly perforated screen, can be removed and the screen coated and exposed again.
The resolution in screen printing depends solely on the mesh width and the thread or wire thickness of the screen. The finer-meshed the screen is, the greater the resolution is. Since cylindrical screens with mesh widths of approx. 120/cm are used for high resolutions and such a mesh fabric is extremely unstable, high-resolution screens can be produced only with small effective printing widths (so-called working widths) of less than 60 cm if sufficient stability of the print roll is to be guaranteed.